Gas geyser operates on LPG fuel is replaced with the Biogas. In gas geyser cold water from tank flows when a hot water tap is opened, then water flow is detected by a sensor and biogas flow is starts through the gas burner. The ignition is needed to start manually by operating gas lighter. The water typically follows a serpentine pattern through the heat exchanger, absorbing as much heat as possible. The heat exchanger is a device which transfers heat from one medium to another.